The present invention relates to multi-layer assemblies of thin films, such as for example, those formed from polysilicon. The thin films have predetermined stress characteristics. The present invention also relates to forming such thin films with desired stress profiles by controlling film formation conditions. By appropriately depositing variously stressed thin films and forming multi-layer assemblies therefrom, a resulting multi-layer assembly may be produced that exhibits a predetermined stress profile. The predetermined distribution of stresses in the multi-layer assembly may be sufficient, if desired, to induce geometrical effects such as curling or arching of the assembly. A wide array of devices and applications are also disclosed that may utilize, or at least be based upon, the present invention.
Polysilicon is one of the most widely used structural materials for microelectromechanical systems (MEMS) and devices. However, when deposited by low-pressure chemical vapor deposition (LPCVD) techniques, polysilicon films typically display high residual stresses and often stress gradients as well. These stresses, particularly when compressive, may cause released devices to bend and buckle, altering their original shapes and degrading their performances. While tensile stresses may promote planarity in doubly clamped designs, such stresses also increase stiffness and cause deformation of asymmetric features. Zero-stress polysilicon thin film structures would be optimal for many applications.
Prior artisans have attempted to produce thin silicon films with reduced stress levels. U.S. Pat. No. 5,753,134 entitled xe2x80x9cMethod for Producing a Layer With Reduced Mechanical Stressesxe2x80x9d to Biebl, is directed to a method for producing a silicon layer having a reduced overall stress value, the layer being composed of two silicon sublayers. The first sublayer and the second sublayer are matched to one another such that the stresses in the two layers substantially compensate each other, and in effect, cancel each other out. However, Biebl requires that one or more auxiliary layers of silicon dioxide be provided between the sublayers. Those auxiliary layers require additional manufacturing or processing operations. Although satisfactory in some respects, a need still exists for an improved multi-layer polysilicon assembly and technique for forming, and particularly for an assembly that does not require the use of intermediate or auxiliary layers.
In addition, a disadvantage often associated with polysilicon films deposited by chemical vapor deposition techniques, pertains to the resulting relatively rough surface of the deposited film. Although approaches are known for producing films having relatively smooth finishes, typically, additional processing steps are necessary or critical process control schemes must be implemented. Accordingly, a need remains for a technique for producing films and multi-layer assemblies of such films having relatively smooth surfaces.
Recent efforts in the field of MEMS have been directed to producing microdevices and microstructures that exhibit a particular geometrical configuration. Often, it is desirable to produce curved or cantilevered geometries. However, given the relative small scale of such geometries, conventional microdevice fabrication techniques have been found to not be suitable, and in many instances, exhibit significant limitations. Accordingly, it would be desirable to provide a technique whereby microdevices and particularly microstructures, could be produced and which would exhibit a specific geometrical arrangement.
The present invention achieves the foregoing objectives and provides in a first aspect, a method for selectively imparting a predetermined cumulative bending moment to a multi-layer assembly of polysilicon thin films. The method comprises providing a multi-layer assembly of polysilicon thin films which has an outermost exposed face. The method also involves then depositing a relatively thin polysilicon layer onto the outermost face under conditions such that the layer will exhibit known residual stresses. The thickness of the layer is controlled during deposition so that a desired cumulative bending moment for the resulting assembly is obtained.
In an additional aspect, the present invention provides a method for producing a multi-layer assembly of polysilicon thin films, such that the assembly has a predetermined overall bending moment. As explained in greater detail herein, the predetermined bending moment may be positive, negative, or zero. The method comprises forming a plurality of polysilicon thin film layers such that each layer exhibits a specific set of characteristics as follows. Each layer has either a devitrified microstructure and an internal tensile stress, or a predominantly columnar microstructure and an internal compressive stress. Each layer further exhibits a microstructure different than that of an adjacent thin film layer. Furthermore, each layer has a thickness and location in the multi-layer assembly such that the moments of each of the thin film layers sum to the predetermined desired overall bending moment.
The present invention further provides multi-layer assemblies formed by each of the foregoing noted methods.
In a further aspect, the present invention provides a multi-layer thin film assembly that includes a substrate upon which are disposed first and second thin films of polysilicon. One of the films has a devitrified microstructure and an internal tensile stress. The second thin film has a predominantly columnar microstructure and an internal compressive stress. The two films are immediately adjacent one another. The multi-layer assembly also includes a third thin film that is disposed on either the first or second thin films. The third film comprises polysilicon and has a thickness of between about 1 nm and about 1000 nm.
In yet an additional aspect according to the present invention, a method for forming a smooth surface from a polysilicon material is provided. In this aspect, the polysilicon smooth surface exhibits a RMS surface roughness value of less than 60 nm. The method comprises providing a substrate, and forming a polysilicon thin film having a devitrified microstructure and an internal tensile stress. The polysilicon thin film is deposited upon the substrate by chemical vapor deposition using silane at a temperature of from about 500xc2x0 C. to about 590xc2x0 C.
Moreover, in a further aspect, the present invention provides a technique for selectively modifying the stress characteristics in a collection of multi-layer polysilicon assemblies, particularly in a manufacturing environment. The method comprises preparing a collection of multi-layer assemblies and then identifying at least one representative sample from that collection. The one or more samples are then analyzed to determine their stress characteristics. Once having identified information of the stresses in the collection of assemblies, one or more relatively thin polysilicon layers are deposited on the assemblies to alter the overall stress characteristics of each of the assemblies.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. It should, however, be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.